This invention relates to an automatic transmission for an automotive vehicle which is equipped with a hydraulic torque converter having direct-coupling means. Among automatic transmissions for automotive vehicles such as automobiles, those equipped with a hydraulic torque converter having a direct-coupling means have been developed. Such torque converter is adapted to directly couple or lock the input side of the torque converter to the output side of the same when hydraulic torque converting action is not required while the vehicle is running, so as to inhibit slippage in the torque converter for efficient driving of the vehicle.
Such direct-coupling of the torque converter is desired to take place from the time the vehicle is running at a low speed, to completely eliminate slippage of the torque converter. However, when the rotational speed of the engine is low, torque produced by the engine fluctuates widely. Therefore, vibrations of the engine are directly transmitted to the vehicle frame if the torque converter is directly coupled or completely locked in such a low vehicle speed region, causing discomfort to the driver. To avoid this, it is necessary to weaken the coupling force of the torque converter to allow small slippage of the torque converter thereby absorbing vibrations of the engine. On the other hand, in a high speed region of the engine, output torque of the engine is large to increase an amount of torque to be transmitted by the torque converter, requiring large direct-coupling force of the torque converter to inhibit slippage and transmission loss of torque through the torque converter.